drift setups have massive understeer because at the beginning an oversteer has to initiated from an understeer. During the understeer, ppl will either attempt to brake drift ( most basic technique) , weight shift or clutch kick, etc... and the rear wheels will spin at a different speed from the front, which will induce into a drift. a overtseer set, it will just snap oversteer, it's very easier to spin out.
I make understeer sets to force myself to weight shift eveytime i go around a corner. When i attend competitions, i have neutral sets so i can be more "forgiving" (not the right word) due to stress, errors, etc